What is Sectional Density?

This is a guest post by Andrew Betts

What is sectional density and why should I care?

Yawn.  Those two words sound incredibly boring. Could we maybe talk about something more exciting like IPv6 subnetting or the subtleties of the EIC allowance in the 2014 tax year vice the 2013 tax year? Okay, it may not spark the passionate emotional response that the time honored 9mm vs. .45 debate elicits, but it’s an important concept for a person to understand when selecting defense ammunition.  Sectional density is a measure of the ratio between the bullet’s weight and length. Why is this important? Let’s have a look at a couple practical examples to answer that question.

Everyone knows that .357 mag 125 gr JHP performs very well. Even when cheap, generic bullets are used, the performance is excellent. This ballistic test of Underwood loaded 125 gr Montana Gold JHP is a perfect example of what to expect from most 125 gr .357 magnum. It penetrates well, expands beautifully, and creates a substantial wound cavity. It’s nothing magical, but it shows excellent performance for a handgun and leaves little to be desired.

Bear in mind that the Montana Gold JHP is not one of the latest generation of bonded wonder bullets. It is a traditional “cup and “core” design and is not marketed for defensive use. The test is not an anomaly, though. 125 gr .357 mag is well known for great performance with a variety of jacketed hollow point bullets. One would reasonably conclude that 125 or 135 gr bullets in 10mm would likewise perform very well, since 10mm produces similar velocities.

Except the 10mm bullet penetrated less than half the depth that the .357 went, despite moving at approximately the same velocity. How could this be? Although it had slightly more weight and slightly more velocity, the 10mm bullet has a diameter of 0.400” and a frontal area of 0.126” while the .357 has a diameter of 0.357” (obviously) and a frontal area of 0.100”. So what?

Bigger is better, right? Well, sort of. Sure, a bigger hole in a bad guy is a good thing, so long as it penetrates deeply enough. That wider frontal area with the same mass behind it means more drag in tissue, which means the bullet comes to a rest sooner. If the bullet comes to a rest before reaching vital organs, incapacitation may take a very long time.

Even enormously fat people don’t have much more than five or six inches of flesh between their mumu and their heart, so why wouldn’t that 10mm 135 gr JHP work famously? It certainly LOOKS more devastating than the .357 shot did. It is true that if a person were to stand perfectly still and let you shoot them in the chest, eight inches of penetration ought to be more than enough to cause rapid incapacitation. The trouble is, people rarely stand perfectly still while at the same time representing a credible threat to the safety of others so that would be called “murder”.

The truth is that if you actually have to fire your weapon, you are now in a highly dynamic and desperate fight for life. Your attacker will likely be moving quickly as will you, and you or he may fire from unusual positions. You could find yourself kneeling, crouching, lying supine, or who knows what? That means that the bullet you fire at your attacker is likely to strike at an oblique angle and have to traverse much more tissue to get to the heart and pulmonary blood vessels. Worse, when people attack other people, they outstretch their arms, either with a weapon, or even just bare fists. It is very common for bullets fired in defense to strike the arms of an attacker first, further increasing the distance a bullet needs to traverse in tissue.

All other factors being equal, bullets with higher sectional density tend to penetrate more deeply. Higher sectional density means a longer bullet, which means a heavier bullet and one with more bearing surface. That is to say that more of the bullet is in contact with the inside of the barrel during firing. The increased friction combined with the higher mass means lower velocity, which can result in less expansion. So we don’t necessarily just want the heaviest bullet that we can find in a given caliber, either. We want the bullet weight that is the best balance of penetration and expansion. That is a complicated issue and worthy an entirely separate article.

Andrew Betts served with the Arizona National Guard for over 12 years, including a tour to Afghanistan.  Visit his YouTube Channel for more great shooting information.

Photo credit – Andrew Betts